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Origins of Large Rate Enhancements in the Nazarov Cyclization Catalyzed by Supramolecular Encapsulation
Author(s) -
Hastings Courtney J.,
Bergman Robert G.,
Raymond Kenneth N.
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201303885
Subject(s) - catalysis , chemistry , protonation , supramolecular chemistry , reaction mechanism , reaction rate , substrate (aquarium) , photochemistry , combinatorial chemistry , organic chemistry , molecule , ion , oceanography , geology
The self‐assembled supramolecular host [Ga 4 L 6 ] 12− ( 1 ; L= N,N ‐bis(2,3‐dihydroxybenzoyl)‐1,5‐diaminonaphthalene) catalyzes the Nazarov cyclization of 1,3‐pentadienols with extremely high levels of efficiency. The catalyzed reaction proceeds at a rate over a million times faster than that of the background reaction, an increase comparable to those observed in some enzymatic systems. A detailed study was conducted to elucidate the reaction mechanism of both the catalyzed and uncatalyzed Nazarov cyclization of pentadienols. Kinetic analysis and 18 O‐exchange experiments implicate a mechanism, in which encapsulation, protonation, and water loss from substrate are reversible, followed by irreversible electrocyclization. Although electrocyclization is rate determining in the uncatalyzed reaction, the barrier for water loss and for electrocyclization are nearly equal in the assembly‐catalyzed reaction. Analysis of the energetics of the catalyzed and uncatalyzed reaction revealed that transition‐state stabilization contributes significantly to the dramatically enhanced rate of the catalyzed reaction.